Ruatabagas and turnips are relatively recent in their modern, high-yielding form. brassica spp. as we know them are almost all modern.
Might sowbread be a starting point? The problem is, of course, that where the crop comes from and how it performs is very important in this context. When you look at Northern Europe from the agronomist's perspective it does seem that it got screwed with having a Mediterranean crop package for a long time. I'm not an expert, but an archeologist explained to me that a big explanation for the cultural explosion of the middle ages was the development of a crop package and argicultural technique really suited to the climate (rye, oats, barley, additionally spelt and buckwheat, with faba beans and brassica varieties to augment, heavy dairy farming and forest pigmast).
Building a high-input, high-intensity crop suited to latitudes like Ireland's, Frisia's or Poland's should certainly shake up the European neolithic or bronze age. It would completely alter the population dynamic, too. instead of a relatively low-yield, extensive form of agriculture augmented weith intensive cultures on the side, you would have dense, concentrated populations spreading out, as it were, organically. No ver sacrum, just the relentless pressure of the teeming multitudes.